Is there a way to connect to ReadyNAS 214 using USB in case of NIC failure

USB is designed as a host/device interface.  The ReadyNAS is a host.  A USB device is generally "dumb", a NAS is anything but.  The only NAS I have ever seen that supports connection via USB requires that you segregate the portion accessible by USB from that accessible by Ethernet.

They designed it that way because that's the proper protocol.  Most of that protocol is handled in the SoC itself.  I don't know if the USB section AL314 in the 214 supports anything other than standard host/device or if the 214 uses another chip for some of the ports that does, but it's not standard that they would.  It's your expectations that are wrong, not a bad design.

Your NIC is likely overheating because it has been partially damaged.  Nobody designs a cooling system for that condition. If the NIC doesn't have a heat sink installed, you might be able to get a bit more time from it by installing one.  But I don't own a 214 nor does Netgear make schematics available to the public, so I can't help you locate it.  On some NAS, some NIC components are on a daughterboard, but they aren't the type of parts that typically overheat.  It's the tranceiver that will.

BTW, one thing that can overstress your NIC is that it is always powered, even when the NAS is "off".  That's so Wake on LAN will work.  If you allow the NAS to get hot when it's "off" (and the fan isn't running) thinking it won't matter, you're wrong.  I don't know if you did that, it's just a possibility.  But it also means that if you want the NIC to cool down, you need to unplug the unit.

Netgear did admit to a problem in early OS versions affecting the 214 NIC (see ReadyNAS-204-Will-not-shut-down-fully-shows-do-exit-9ec )  If you ran a version earlier than 6.9.4 for a long time, that could also be the cause of the overstress.  But getting Netgear to admit that may have permanently damaged you NAS and RMA it if it's past warranty is not likely to happen, I suspect.